store.sync call back does not work on running it second time

Problem is that if I don't hide or close my input form, and try to input twice. The first time, it will run the callback functions, however, the second time, it will not run the success or failure at all.

I noticed that there is a 'callback' function except the success and failure, so is it possible store.sync will call the 'callback'? Or is the 'callback' the default function when store finishes its work?